Hydraulic braking system especially for motor vehicles

ABSTRACT

A braking system is disclosed that has a manually-actuatable master cylinder having two pressure chambers. A first pressure chamber hydraulically controls a flow control valve through which the pressure medium flows during servo-braking in order to adjust its regulating cross-section to produce a defined dynamic pressure that is applied to a first piston/cylinder arrangement, connected to the flow control valve, of a brake application element to produce a braking force. A second pressure chamber can be connected to a second piston of a brake application element so that a braking force can be applied to the brake application element if the servo-force fails. Also, instead of the second piston/cylinder arrangement, a separate separator piston/cylinder arrangement can be provided that is inserted between the second pressure chamber and the piston/cylinder arrangement. As a result, sufficient braking force is produced both in servo-force braking and if the servo-force fails. Also, an appropriate response by the braking forces to the master cylinder is effected so that there is always good actuating feedback.

CROSS-REFERENCE TO RELATED APPLICATIONS

This case is a divisional of application Ser. No. 08/707,886 filed Sep.9, 1996, now U.S. Pat. No. 5,700,067.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a hydraulic braking system.The invention is especially a hydraulic power-assisted or power brakingsystem for motor vehicles in which the servo force on the brakeapplication element is applied by dynamic pressure produced when apressure medium flows through a control valve, said dynamic pressurebeing able to be adjusted in a defined manner by controlling the controlvalve using an actuating device.

2. Description of the Related Art

Today, hydraulic braking systems for motor vehicles are conventionallydesigned as power-assisted or power braking systems. In a power-assistedbraking system, the energy required to produce the braking force derivesfrom the physical force of the vehicle driver and from one or moreenergy supply devices, i.e., during the braking process, some portion ofthe braking force is applied directly to the brake application elementthrough a master cylinder connected to a manually-actuatable brakepedal, while the rest of the braking force is applied as servo-force(from a hydraulic pump, for example). In contrast, in a power brakingsystem, the energy required to produce the braking force derives fromone or more energy supply devices, with the exception of the physicalforce of the vehicle drive; in other words, the master cylinderconnected to the brake pedal does not apply the braking force directlyto the brake application element, but rather it controls the servo-forceacting on the brake application element.

A servo-brake for a power braking system is known from DE-AS 1 134 904;in it the servo-force is produced by a pressure circulating system. Inaddition, connected at the output of a hydraulic pump, the servo-brakehas a pressure chamber connected to the brake application element and anoverflow chamber attached to a reservoir, said overflow chamber beingconnected to the pressure chamber by a flow control valve. The flowcontrol valve has a valve regulator body that together with a bushingfixed in the power braking system defines the valve passage area of theflow control valve and is connected to the brake pedal, so that thevalve passage area of the flow control valve can be adjusted dependingon the position of said brake pedal in order to produce dynamic pressurein the pressure chamber that is proportionate to the travel of the brakepedal. This dynamic pressure is forwarded to the brake applicationelements through a piston in the pressure chamber. The piston has aflow-through opening.

If the hydraulic pump fails, it is possible to produce the requiredresidual braking force by mechanically controlling the brake circuit. Anadditional valve element is provided for this purpose that can beapplied with the valve regulator body, if the valve regulator body ispushed far enough into the bushing so that the flow control valve isclosed. The additional valve element can now be displaced through thevalve regulator body against the force of a restoring spring in thedirection of the piston in the pressure chamber in order to close theflow-through opening in the piston so that the brake applicationelements can be controlled by the piston.

This is disadvantageous in that integration of the residual brakingfunction into the servo-brake is relatively difficult to achieve inaccordance with this state of the art, given the high number ofcomponents and large dimensions. Furthermore, there is the problem thatif the servo-force fails, the relatively large stroke of the flowcontrol valve has to be travelled in order to control the brake circuitto produce the required residual braking force, so that if theservo-force fails, the brake actuates unusually late, which can lead todangerous situations.

These types of servo-brakes that work with a flow control valve inaccordance with the principle of dynamic pressure are also known forpower-assisted braking systems in accordance with DE-PS 1 180 259. Theservo-brake disclosed in DE-PS 1 180 259 essentially differs from theservo-brake described above in that the piston in the pressure chamberis actuated by the valve regulator body connected to the brake pedal,even during normal braking, while instead of the additional valveelement, a non-return valve is attached in the flow-through opening ofthe piston. The non-return valve is pre-stressed in its closed positionagainst the actuating direction of the valve regulator body andunblocking the path for the pressure medium only in the direction of thebrake application element.

Furthermore, an additional pressure area containing an additional pistonis provided. The area is attached between the valve regulator body ofthe flow control valve and the piston in the pressure chamber in orderto apply to the valve regulator body, and therefore to the brake pedal,a reaction force proportionate to the dynamic pressure. This response ofthe dynamic pressure against the flow control valve to the brake pedalis required in order to enable responsive actuation of the brake pedalthat reflects the braking pressure in each of the brake applicationelements.

In this system, regardless of whether immediate brake response isprovided by direct mechanical action on the piston in the pressurechamber, even if the servo-force fails, it can be considered adisadvantage of this servo-brake that during the selected directmechanical action on the piston, the response of the dynamic pressureagainst the flow control valve to the brake pedal requires relativelygreat expense in terms of technical devices. In other words, it requiresa large number of components and a great deal of room for the valve. Inaddition, when the brake pedal is actuated, the mass of the additionalpiston has to be moved, as well, and the frictional force between theadditional piston and the interior wall of the additional pressure areahas to be overcome, which further increases the actuating force requiredto produce the necessary residual braking force on the brake pedal inthe event that the servo-force fails.

Furthermore, DE-PS 1 037 287 illustrates a servo-brake for a power brakesystem in which the brake circuit is separate from the servo-circuit.This system also works with a flow control valve using the principle ofdynamic pressure. Extending through the overflow chamber is a valveregulator body, one end of which can be hydraulically controlled by themaster cylinder, which is connected to the brake pedal. The other endprojects into the pressure chamber of the servo-circuit. The valveregulator body is provided with a center-drilled pocket on the end thatprojects into the pressure chamber of the servo-circuit; there is across-hole at the base of the pocket that connects the pressure chamberof the servo-circuit to the overflow chamber. The flow control valve isformed by the end of the valve regulator body that projects into thepressure chamber of the servo-circuit and the base of a plunger pistonpositioned concentric with the valve regulator body. The plunger pistonextends sealed through a separating wall between the pressure chamber ofthe servo-circuit and the pressure chamber of the brake circuit.

Although this system has the advantage that, by separating theservo-circuit from the brake circuit, a servo-source having, forexample, a different operating medium for the servo-circuit alreadypresent in the vehicle can be used, there are still problems in terms ofthe path-dependant control of the valve passage area of the flow controlvalve. On the one hand, the effective hydraulic area of the valveregulator body on the master cylinder side is too small to producehydraulically the required residual braking force through the mastercylinder with proportionate actuating force on the brake pedal if theservo-force fails. On the other hand, the effective hydraulic area ofthe valve regulator body on the master cylinder side is so large thatduring normal braking it requires a great deal of pressure medium or alengthy stroke at the master cylinder with correspondingly higheractuating force on the brake pedal to adjust the valve passage area ofthe flow control valve and thereby to adjust the servo-force. Inaddition, during normal braking, the regulating process adjustment thatoccurs when the valve regulator body is displaced by pressure throughthe master cylinder on the plunger piston at the valve passage area ofthe flow control valve leads to a situation in which the brake pedalsubjected to the actuating force gives way, which is characterized asthe brake pedal "running away." As a result, it can be stated that bothduring normal braking, with servo-support, and when the servo-force ofthe brake pressure affecting brake application elements fails, there isnot sufficient sensitivity in the brake pedal, so that actuation andpedal response are not good. Furthermore, known from DE 39 05 044 A1 orDE 43 22 292 A1, for example, are pressure modulators for power-assistedor power braking systems, by means of which the pressure of a hydraulicpump can be regulated in accordance with the principle of reactionpressure in order to produce a servo-force proportionate to actuatingforce. Such a pressure modulator is inserted into the servo-circuit ofthe braking system and has a valve housing in which a sliding valvepiston is arranged. One side of said valve piston borders a controlchamber that is hydraulically connected to the master cylinder attachedto brake pedal, while the other side of said valve piston borders apressure chamber. The pressure chamber is connected to a hydraulic pumpby a pressure line and has an outlet that is connected to a reservoirand that can be closed using a seat on the valve piston. The pressureline connecting the pressure chamber to the hydraulic pump has a branchthat is connected hydraulically to the brake application elements (inthe case of DE 39 05 044 A1) or that leads to a servo-brake (in the caseof DE 43 22 292 A1).

When operating a braking system of this design, actuation of the brakepedal produces pressure in the control chamber of the pressuremodulator; the pressure pushes the valve piston so that the seat of thevalve piston closes the pressure chamber outlet. As a consequence,pressure builds up at the hydraulic pump outlet, this pressure alsoacting on the brake application elements, i.e., in the servo-brake,through the branch. As soon as the pressure at the hydraulic pump outlet(and thereby in the pressure chamber of the pressure modulator) is equalto the pressure in the control chamber, the valve piston is pressed backfrom the pressure chamber outlet so that excess pressure medium is fedinto the reservoir. A regulating process is initiated that determinesthe pressure at the hydraulic pump outlet depending on the pressure inthe control chamber.

One disadvantage of these systems is that, in the case of DE 39 05 044A1, regulating the servo-pressure using the pressure modulator bypushing the valve piston away from the pressure chamber outlet whiledecreasing the volume of the control chamber affects the master cylinderso that regulating pressure surges can be detected on the brake pedal asa result of the regulating pressure. Also, in the case of DE 43 22 292A1, the absence of retroactive effects resulting from the regulatingprocess in the pressure modulator can only be provided at great expensein terms of technical devices and using a liquid chamber in theservo-brake and additional valves.

Finally, use of electro-magnetically controlled ball check valves inpressure modulators for hydraulic anti-lock braking systems (ABS) isknown in principle from DE 36 03 074 C2 and DE 37 02 573 A1. In thesepressure modulators, the ball check valve functions as an on/off valve;the ball valve of which can be pushed indirectly by anelectro-magnetically adjustable pressure release piston of the pressuremodulator or directly by its own electro magnets, provided for thispurpose, in order to interrupt the connection between the mastercylinder and the wheel brake cylinder. This means that during ABSoperation, braking pressure in the wheel brake cylinder can be regulatedby the pressure relief piston regardless of the pressure in the mastercylinder.

To summarize, it can be stated that the known power-assisted and powerbraking systems in which the servo-force is produced by a hydraulic pumpin accordance with the principles of dynamic and reaction pressure arein need of improvement in terms of satisfactory response to the brakepedal from the braking pressure occurring in the brake applicationelements, appropriate actuation force requirement on the brake pedalwhen there is sufficient braking force on the brake applicationelements, and regulation processes in the servo-circuit with noretroactive effects.

SUMMARY OF THE INVENTION

Therefore, with respect to the state of the art described above, thepresent invention is based on the object of providing asimply-constructed, hydraulic power-assisted or power braking system toproduce sufficient braking force on the brake application elements,along with good pedal responsiveness, during normal braking, even if theservo-force has failed. This object is achieved by a hydraulic brakingsystem, especially for motor vehicles, including an actuating device anda master cylinder connected to the actuating device. The master cylinderincludes a first pressure chamber and a second pressure chamber. Thesystem has a flow control valve, through which flows a pressure mediumduring the servo-force braking, hydraulically connected to the firstpressure chamber so that the first pressure chamber hydraulicallycontrols the flow control valve by adjusting its regulatingcross-section to produce a defined dynamic pressure. The system alsoincludes a brake application element constructed and arranged to producea braking force. The brake application element includes a firstpiston/cylinder arrangement and a second piston/cylinder arrangement.The first piston/cylinder arrangement is hydraulically connected to theflow control valve and is responsive to the defined dynamic pressurereceived from the flow control valve. The second piston/cylinderarrangement is hydraulically connected to the second pressure chamber toproduce a braking force to the brake application element if theservo-force braking fails.

In accordance with an embodiment of the present invention, a hydraulicbraking system is provided. The system includes a master cylinderconnected to an actuating device. The master cylinder has two pressurechambers. Of the two pressure chambers, the first hydraulically controlsa flow control valve during servo-force braking in order to adjust itsregulating cross-section to produce a defined dynamic pressure that isapplied to a first piston/cylinder arrangement, hydraulically connectedto the flow control valve, of a brake application element to produce abraking force. The second pressure chamber of the master cylinder can behydraulically connected to a second piston/cylinder arrangement of abrake application element so that a braking force can be applied to thebrake application element if the servo-force fails.

In accordance with another embodiment of the present invention, ahydraulic braking system having a master cylinder attached to anactuating device, wherein the master cylinder has two pressure chambersis provided. The first pressure chamber hydraulically controls a flowcontrol valve through which flows a pressure medium during servo-forcebraking in order to adjust its regulating cross-section to produce adefined dynamic pressure that, in order to produce a braking force, isapplied hydraulically in servo-force braking to a piston/cylinderarrangement of a brake application element that is connected to the flowcontrol valve which is connected hydraulically to a servo-pressurechamber of a separator piston/cylinder arrangement. The second pressurechamber of the master cylinder can be hydraulically connected to theactuating pressure chamber, separated from the servo-pressure chamber bya separator piston, of a separator piston/cylinder arrangement, so thata braking force can be applied to the brake application element throughthe separator piston if the servo-force fails.

The invention provides functional separation of the servo-circuit fromthe manually adjustable brake circuit by two separate piston/cylinderarrangements in the brake application element and the combination of onepiston/cylinder arrangement in the brake application element and anadditional or separate separator piston/cylinder arrangement, as well ashydraulic control of the flow control valve to produce the dynamicpressure when a flow is forced through the regulating cross-sectionproduces sufficient braking force on the brake application element and aproportionate response from the braking forces to the actuating device,both during servo-force braking and if the servo-force has failed, sothat there is always good actuating or pedal responsiveness.

The hydraulic braking systems designed in accordance with the inventionas described above have in particular the advantage of great flexibilityin terms of space. In other words, the individual components (like theflow control valve, the hydraulic pump, and the separatorpiston/cylinder arrangement) are not tied to a specific position in thevehicle (e.g., the brake pedal), but rather can also be arranged atanother location in the vehicle where space is more readily available orfor convenience.

In addition, additional components can be easily integrated into thefunctionally-divided hydraulic braking system to satisfy additionalfunctions (like anti-lock braking systems, traction control (ASR), andvehicle dynamics regulation system). Finally, the expense of technicaldevices decreases significantly compared to the known art describedearlier because the individual components that satisfy only a fewfunctions can be designed more simply as a result of the functionalseparation addressed above.

The second described embodiment of the hydraulic braking system isespecially suitable for converting existing braking systems, sinceconventional single-chamber brake calipers and wheel brake cylinders canbe controlled hydraulically by drum brakes. In addition, duplicate brakelines need not be laid for every caliper, so that the design of thisbraking system is particularly simple in terms of technical devices.Finally, a switch from servo-force braking operations to power-assistedor emergency operation of the braking system by the separatorpiston/cylinder arrangement, depending on the effective hydraulic areasat the separator piston, and by the movement of the separator piston canbe automatically and advantageously recognized (e.g., by a directionswitch) and reported to the driver.

In an embodiment of this invention, a brake application element iscreated that is particularly adapted to the first described embodimentof the hydraulic braking system wherein the brake application elementhas two piston/cylinder arrangements. Because of the advantageousconcentric arrangement of the two piston/cylinder arrangements, thebrake application element is no larger than a conventional brakecaliper.

The features of the embodiments of this invention create a modulardesign that permits simple expansion of the hydraulic braking system toABS, ASR, and/or a vehicle dynamics control system, wherein theadditional regulation (in the case of ABS) can essentially be conductedwith no retroactive effects so that good pedal responsiveness is stillprovided. In particular, the hydraulic braking system having ABS isdesigned advantageously because there is no need to provide a reliefvalve to functionally separate the manually-controlled brake circuitfrom the power brake circuit when the anti-lock braking system is inoperation. On the contrary, this function is satisfied by the separatorpiston/cylinder arrangement in the circuit as given.

Advantageous embodiments of the separator piston/cylinder arrangementare also provided. For example, an embodiment of the separatorpiston/cylinder arrangement has a sensor for sensing the separatorpiston movement, which makes it easy to determine whether the hydraulicbraking system is switched to emergency operation. The signal emitted bythe sensor can then, for example, be used to display the operating modeof the hydraulic braking system.

In accordance with another embodiment, the flow control valve isdesigned advantageously as a ball check valve, so that the regulatingcross-section can be adjusted reliably and precisely to produce adefined dynamic pressure. Furthermore, the use of a ball check valve iscost-effective.

The present invention is explained in more detail in the following,using preferred embodiment examples and referring to the drawing,identical or similar parts having identical numerical labels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a principal illustration of a first embodiment example of thebraking system in accordance with the invention, designed as apower-assisted braking system, having parallel arrangement of twopiston/cylinder arrangements in the brake caliper.

FIG. 2 is a partial sectional view of a preferred brake caliper for thebraking system in accordance with FIG. 1, having a cylindrical pistonthat forms both the piston of the one piston/cylinder arrangement andthe cylinder of the other piston/cylinder arrangement.

FIG. 3 is a principal illustration of a second embodiment example of thebraking system in accordance with the invention, which in contrast tothe first embodiment example has additional components for an anti-lockbrake system (ABS).

FIG. 4 is a principal illustration of a third embodiment example of thebraking system in accordance with the invention, which in contrast tothe first and second embodiment examples has additional components for atraction control system (ASR) and a vehicle dynamics control system.

FIG. 5 is a principal illustration of a fourth embodiment example of thebraking system in accordance with the invention, which is designed as apower brake system having a separator piston/cylinder arrangementbetween a manually-controllable brake circuit and a servo-circuit andalso having only one piston/cylinder arrangement in the brake caliper.

FIG. 6A is a sectional view of a first variant of the separatorpiston/cylinder arrangement illustrated in FIG. 5.

FIG. 6B is a sectional view of a second variant of the separatorpiston/cylinder arrangement illustrated in FIG. 5.

FIG. 7 is a principal illustration of a fifth embodiment example of abraking system in accordance with the invention, which in contrast tothe first embodiment example has additional components for an anti-lockbrake system (ABS).

FIG. 8 is a principal illustration of a first variant of the fifthembodiment example.

FIG. 9 is a principal illustration of a second variant of the fifthembodiment example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with FIG. 1, an embodiment of a hydraulic power-assistedbraking system of the present invention has a brake pedal 2 serving asan actuating device and connected to a master cylinder 4 (e.g., a tandemmain cylinder) that has two pressure chambers 6, 8. Also provided is abrake caliper 10 acting as a brake application element that is providedwith two piston/cylinder arrangements 12, 14. The first pressure chamber6 of the master cylinder 4 acts as the hydraulic control of a flowcontrol valve 16 in which a pressure medium is forced to flow duringservo-supported braking and which is hydraulically connected to thefirst piston/cylinder arrangement 12 of the brake caliper 10. Theregulating cross-section of the flow control valve 16 is hydraulicallyadjusted by the first pressure chamber 6 of the master cylinder 4 toproduce a defined dynamic pressure that acts on the firstpiston/cylinder arrangement 12 of the brake caliper 10 to produce abraking force. The second pressure chamber 8 of the master cylinder 4 isconnected to the second piston/cylinder arrangement 14 of the brakecaliper 10 so that a braking force can be produced on the brake caliper10 if the servo-support fails.

A piston rod 18 connects the brake pedal 2 to the first piston 20 of themaster cylinder 4 which hydraulically defines the first pressure chamber6 on the brake pedal side. Between the first pressure chamber 6 and thesecond pressure chamber 8 is a second piston 22 that hydraulicallyseparates the first and second pressure chambers 6, 8 from each other.The first and second pressure chambers 6, 8 are each connected to abalance tank 26 by a port 24. The first pressure chamber 6 of the mastercylinder 4 is connected by a control line 28 to a control chamber 30 ofthe flow control valve 16, which is designed as a three-chamber 2/2 ballcheck valve having hydraulic control that has an overflow chamber 32 anda pressure chamber 34 in addition to the control chamber 30. Arranged inthe overflow chamber 32 is a valve regulator body 36 in the form of ametal ball that can be mechanically actuated by a force from a controlpiston 40 that extends sealed through a wall 38 between the controlchamber 30 and the overflow chamber 32 and that can be moved axially.The valve regulator body 36 can only be actuated by the force ofpressure from the control piston 40, since the valve regulator body 36and the control piston 40 are two separate components. Provided in awall 42 between the overflow chamber 32 and the pressure chamber 34 is athrough hole 44 that hydraulically connects the overflow chamber 32 andthe pressure chamber 34 to each other. Attached at the overflow chamberside of the through hole 44 is a ring-shaped sealing seat 46; togetherwith the valve regulator body 36 it defines a valve passage area 48, theflow-through cross-section of which corresponds to the regulatingcross-section of the flow control valve 16. Attached in the pressurechamber 34 is a restoring spring 50 that extends through the throughhole 44 and presses the valve regulator body 36 against the controlpiston 40. If there is no control pressure in the control chamber 30,the control piston 40 is held in its starting position by the restoringspring 50 through the valve regulator body 36, the valve passage are 48being opened to its maximum (through-null position of the flow controlvalve 16).

The pressure chamber 34 of the flow control valve 16 is connectedhydraulically by a pressure line 52 to the output of a hydraulic pump54, the inlet of which is connected to a reservoir 58 for the pressuremedium, preferably brake fluid, by an inlet line 56. A non-return valve60 inserted into the pressure line 52 is pre-stressed in the directionof the hydraulic pump 54. The overflow chamber 32 of the flow controlvalve 16 is connected to the reservoir 58 by a return line 62. Apressure maintenance valve 64 inserted into the return line 62 ispre-stressed in the direction of the overflow chamber 32.

Branching off from the pressure line 52 between the non-return valve 60and the pressure chamber 34 of the flow control valve 16 is a pressureline 66 that is connected to the pressure chamber 68 of the firstpiston/cylinder arrangement 12 of the brake caliper 10 so that thepiston 70 of the first piston/cylinder arrangement 12 can be acted uponby the dynamic pressure produced by the hydraulic pump 54 and controlledby the flow control valve 16.

The second pressure chamber 8 of the master cylinder 4 is attacheddirectly to the pressure chamber 74 of the second piston/cylinderarrangement 14 of the brake caliper 10 by a pressure line 72 so that thepiston 76 of the second piston/cylinder arrangement 14 can be acted uponby the pressure produced in the second pressure chamber 8 by the secondpiston 22 of the master cylinder 4 as a result of downward pressure onthe brake pedal 2.

The brake caliper 10 (a floating caliper in the illustration) isprovided with brake linings 78 that are pressed against a brake disk 80by the piston 70 of the first piston/cylinder arrangement 12 or by thepiston 76 of the second piston/cylinder arrangement 14 in a known mannerwhen pressure acts upon the first/second piston/cylinder arrangement 12,14. In the illustration, the first and second pressure/cylinderarrangements 12, 14 are arranged next to or parallel to each other, butthey can also be arranged concentrically or on opposing sides of thebrake disk 80 of the brake caliper 10, for example.

Finally, a pedal switch 82 that can be mechanically actuated by thebrake pedal 2 and/or an electro-hydraulic manometric switch 84 insertedinto the control line 28 is/are provided; the function of these will beexplained in the following.

As a result, the hydraulic pump 54, the pressure line 52, the flowcontrol valve 16 connected to the reservoir 58 by the return line 62,the pressure line 66, and the first piston/cylinder arrangement 12 ofthe brake caliper 10 form a power brake circuit A that is controlled bya hydraulically-separate control circuit C comprising the piston 20 ofthe master cylinder 4, the first pressure chamber 6 of the mastercylinder 4, the control line 28, the control chamber 30 of the flowcontrol valve 16, and the control piston 40 of the flow control valve16. The second piston 22 of the master cylinder 4, the second pressurechamber 8 of the master cylinder 4, the pressure line 72, and the secondpiston/cylinder arrangement 14 of the brake caliper 10 form a brakecircuit that is independent from power brake circuit A and that can becontrolled manually by the brake pedal 2.

At this point, it should be remarked that, for the sake of simplicity,FIG. 1 merely illustrates a basic embodiment of the power-assistedbraking system having a brake caliper 10 for a wheel. Thepiston/cylinder arrangements of additional brake calipers are attachedto the first piston/cylinder arrangement 12 in the brake caliper 10 bythe branch 66' from the pressure line 66, which connects the pressureline 52 between hydraulic pump 54 and pressure chamber 34 of the flowcontrol valve 16, or by the branch 72' from the pressure line 72 betweensecond pressure chamber 8 of the master cylinder 4 and secondpiston/cylinder arrangement 14 of the brake caliper 10, as appropriate.Thus, on a four-wheel motor vehicle it is conceivable that only two offour brake calipers are each provided with two piston/cylinderarrangements that are acted upon by power brake circuit A or brakecircuit B as described above, while the other two brake calipers eachhave only one piston/cylinder arrangement that is acted upon by powerbrake circuit A. Other combinations are also possible, commensurate withthe requirements of each situation. This also applies to the embodimentexamples illustrated in FIGS. 3 and 4.

The functioning of the first embodiment example is described in thefollowing. Downward pressure on the brake pedal 2 pushes the pistons 20,22 of the master cylinder 4 in the pressure chambers 6, 8 in FIG. 1 tothe right and closes the ports 24 to the balance tank 26 so thatpressure in the pressure chambers 6, 8 is proportional to the actuatingforce on the brake pedal 2. This pressure now appears as controlpressure in control circuit C for power brake circuit A through thecontrol line 28 in the control chamber 30 of the flow control valve 16.In brake circuit B, this pressure is applied through the pressure line72 in the pressure chamber 74 of the second piston/cylinder arrangement14 of the brake caliper 10.

In control circuit C, control pressure in the control chamber 30 of theflow control valve 16 pushes the control piston 40 in FIG. 1 to theright and thereby presses the valve regulator body 36, which is adjacentto the control piston 40 because of the force of the restoring spring50, in the direction of the sealing seat 46. At the same time, thehydraulic pump 54 is started either by the pedal switch 82 activated bythe brake pedal 2, or, if a pre-determined signal pressure in controlcircuit C is exceeded, by the electro-hydraulic manometric switch 84,for which the pedal switch 82 or the electro-hydraulic manometric switch84 delivers an electric signal that starts an electro-motor (not shown)drive-connected to the hydraulic pump 54 or that engages anelectro-magnetic coupling (not shown) that connects the hydraulic pump54 to a rotating shaft of the motor or wheel of the motor vehicle. Thehydraulic pump 54 could also however be operated throughout the entireduration of vehicle usage.

Now the hydraulic pump 54 draws the pressure medium out of the reservoir58 through the inlet line 56 and conveys it through the non-return valve60 and the pressure line 52 into the pressure chamber 34 of the flowcontrol valve 16. The pressure medium flows from the pressure chamber 34through the valve passage area 48 into the overflow chamber 32 and fromthere through the return line 62 and the pressure maintenance valve 64back into the reservoir 58. Since when the brake pedal 2 is firstactuated the control piston 40 of the flow control valve 16 andtherefore the valve regulator body 36 are only pushed slightly in thedirection of the sealing seat 46 so that the valve passage area 48 isalmost completely open, the hydraulic pump 54 circulates the pressuremedium through the flow control valve 16 with essentially no pressure.

The valve regulator body 36 is now pushed against the force of therestoring spring 50 further in the direction of the sealing seat 46 bythe hydraulically-actuated control piston 40, and it is pushed with aforce equal to the product of the control pressure produced in thecontrol chamber 30 by the master cylinder 4 and the effective hydraulicarea of the control piston 40. As the valve regulator body 36 approachesthe sealing seat 46, the valve passage area 48 decreases, whichdecreases the flow-through cross-section for the pressure mediumcirculated by the hydraulic pump 54 through the pressure line 52 and thepressure chamber 34. As a result, dynamic pressure, produced in powerbrake circuit A in the circulating direction of the pressure medium infront of the valve passage area 48, transmits through the pressurechamber 34, the pressure line 52, and the pressure line 66 up to thepressure chamber 68 of the first piston/cylinder arrangement 12 of thebrake caliper 10, so that a braking force is applied to the brake disk80 by the piston 70 of the first piston/cylinder arrangement 12 and bythe brake lining 78.

The dynamic pressure produced in front of the valve passage area 48essentially depends on the volume flow rate of the hydraulic pump 54 andthe flow-through resistance of the flow control valve 16, but can beaccurately estimated as proportional to the dynamic pressure occurringin the control chamber 30 of the flow control valve 16. Theproportionality factor between this control pressure and the dynamicpressure produced in front of the valve passage area 48 is determined bythe ratio of the effective hydraulic area of the control piston 40 inthe control chamber 30 to the effective hydraulic area of the valveregulator body 36 at the sealing seat 46. Appropriate dimensioning ofthese effective areas on the one hand permits the desired pressureintensification in power brake circuit A to be adjusted. On the otherhand, the reaction force applied through the effective hydraulic area ofthe valve regulator body 36 can be adjusted by the dynamic pressure atthe valve regulator body 36 in such a way that a proportionate responsefrom the dynamic pressure to the brake pedal 2 results through thepressure columns of control circuit C between the control piston 40 ofthe flow control valve 16 and the first piston 20 of the master cylinder4 in order to provide good pedal responsiveness and thereby to enableresponsive actuation of the brake pedal 2. Since the valve passage area48 of the flow control valve 16 is continuously moved and is notcompletely closed against the dynamic pressure during servo-supportedbraking, no regulating pressure surges occur that could retroactivelyaffect the brake pedal 2.

As described above, the pressure produced in brake circuit B in thesecond pressure chamber 8 of the master cylinder 4 by pressing the brakepedal 2 downward occurs simultaneously in the pressure chamber 74 of thesecond piston/cylinder arrangement 14 of the brake caliper 10. Thispressure thereby likewise produces a braking force on the brake disk 80by the piston 76 of the second piston/cylinder arrangement 14 and by thebrake lining 78. The reaction force produced on the piston 76 of thesecond piston/cylinder arrangement 14 is reported back through thepressure column between the piston 76 and the second piston 22 of themain cylinder 4 and through the pressure column between the secondpiston 22 and the first piston 20 of the master cylinder 4, and combinedwith the reaction force in control circuit C, to the brake pedal 2 andenable responsive actuation of the brake pedal 2 that reflects theactual braking pressure in the brake caliper 10.

From the above description it becomes clear that if, for instance, thehydraulic pump 54 or fails or if there is a leak in power brake circuitA, the required residual braking force can be applied to the brake disk80 manually with proportionate response to the brake pedal 2 throughbrake circuit B, which has been functionally uncoupled from brakecircuit A.

If the braking pressure is now to be reduced in servo-supported brakingoperations, releasing the brake pedal 2 decreases the control pressurein control circuit C and the control piston 40 of the flow control valve16 and the valve regulator body 36 return to their original positionwhile the valve passage area 48 enlarges, whereby it is subjectedagainst the decreasing control pressure in the control chamber 30through the valve regulator body 36 by the dynamic pressure, decreasingas a consequence of the enlargement of the valve passage area 48, of thecirculating pressure medium in the pressure chamber 34 and the force ofthe restoring spring 50. If the pre-determined signal pressure incontrol circuit C at the electro-hydraulic manometric switch 84 is notexceeded, the hydraulic pump 54 and its drive are switched off andcirculation of the pressure medium ends. In brake circuit B, the brakingpressure in the pressure chamber 74 of the second piston/cylinderarrangement 14 also naturally decreases as a result of the brake pedal 2being released.

FIG. 2 illustrates a brake caliper 10 for the braking system having thepreferred design of the two piston/cylinder arrangements 12, 14, whichcan also be used in the first through third embodiment examples.

In accordance with FIG. 2, designed in a housing 86 of the brake caliper10 is an annular chamber 88 that includes a slidable pot-shaped annularpiston 90. The annular piston 90 is adjacent to a cylindrical pivot 92of the housing 86, which with the base 94 of the annular piston 90 formsa cylindrical chamber 96. Both the annular chamber 88 and thecylindrical chamber 96 are provided with a connection (not shown) sothat they can be attached to the pressure line 66 of power brake circuitA or to the pressure line 72 of brake circuit B.

The cylindrical chamber 96 is hydraulically sealed against the annularchamber 88 by means of a sealing element 98 that is contained in achannel 100 in the exterior circumferential surface 102 of the pivot 92,but that could also be fixed to the interior surface of the annularpiston 90. The annular chamber 88 is sealed by a sealing element 104that is contained in a channel 106 in the interior circumferentialsurface 108 of the housing.

The annular piston 90 is connected at its end section 110 projecting outof the housing 86 to the housing 86 by an elastic collar 112 thatprevents brake dust from penetrating between the exterior circumferenceof the annular piston 90 and the interior circumferential 108 of thehousing. The annular piston 90 works through its end section 110 withthe brake linings (not shown) in a manner known in and of itself toapply a braking force to a brake disk (not shown).

As a result, the housing 86 together with the annular piston 90 formsthe first and second piston/cylinder arrangement 12, 14 of the brakecaliper 10 in a very compact construction so that the two-chamber brakecaliper 10 does not require more space than a conventional singlechamber brake caliper. Depending on the design of the power-assistedbraking system, either the annular chamber 88 or the cylindrical chamber96 is connected to power brake circuit A by the pressure line 66, whileeach of the other chambers is attached to brake circuit B by thepressure line 72.

In accordance with the second embodiment example illustrated in FIG. 3,the power-assisted braking system in accordance with FIG. 1 is providedwith additional components in order to make an anti-lock brake system(ABS) possible. The parts corresponding to the parts in FIG. 1 arelabelled with the same reference numbers and are not re-explained in thefollowing. FIG. 3 illustrates the power-assisted braking device with ABSin its non-actuated mode.

In principle, an anti-lock brake system ensures that if a certaindeceleration threshold is exceeded at a braked wheel, the wheel brakingpressure is decreased until it falls below a second decelerationthreshold at this wheel. It can be necessary to decrease the wheelbraking pressure to zero. Then the wheel braking pressure is increasedagain until either the affected wheel is overbraked again or the brakingpressure specified by the driver is achieved.

Therefore, in accordance with FIG. 3, manually-controlled brake circuitB is provided a relief valve 114 (in the illustration, anelectro-magnetically actuatable 3/2 directional control valve) that isinserted into the pressure line 72 between the master cylinder 4 and thesecond piston/cylinder arrangement 14 of the brake caliper 10 and thatis connected to the reservoir 58 by a relief line 116. The relief valve114, occurring only once in the system, connects the pressure chamber 74of the second piston/cylinder arrangement 14 to the second pressurechamber 8 of the master cylinder 4 or alternatively to the reservoir 58,and is pre-stressed in its position connecting the pressure chamber 74of the second piston/cylinder arrangement 14 to the second pressurechamber 8 of the master cylinder 4.

Power brake circuit A is provided two on/off valves 118, 120 for eachwheel to be controlled (in the illustration, electro-magneticallyactuatable 2/2 directional valves with flow-through and block settings).The first on/off valve 118 is inserted into the pressure line 66 to thefirst piston/cylinder arrangement 12 of the brake caliper 10 andpre-stressed in such a way that in its normal position it connects thepressure line 52 between the hydraulic pump 54 and the pressure chamber34 of the flow control valve 16 to the pressure chamber 68 of the firstpiston/cylinder arrangement 12. Branching off between the first on/offvalve 118 and the pressure chamber 68 of the first piston/cylinderarrangement 12 is a line 122 that opens into a collective return line124 connected to the reservoir 58 or that is connected directly to thereservoir 58. The second on/off valve 120 is inserted into the line 122and is pre-stressed in its closed position.

If braking is introduced through the brake pedal 2 during ABS operationof the hydraulic power-assisted braking system, sensor technology (notshown) recognizes that the braking pressure applied in the brake caliper10 through power brake circuit A or brake circuit B is locking thebraked wheel (not shown). The braking pressure is now adjusted by theelectro-magnetic control of the valves 114, 118, and 120 as appropriate.In addition, the relief valve 114 is switched to its position connectingthe pressure chamber 74 of the second piston/cylinder arrangement 14 tothe reservoir 58 so that the pressure between the second pressurechamber 8 of the master cylinder 4 and the relief valve 114 is locked inwhile the braking pressure in the pressure chamber 74 of the secondpiston/cylinder arrangement 14 is decreased to zero by the relief line116 to the reservoir 58.

In power brake circuit A, the braking pressure in the pressure chamber68 of the first piston/cylinder arrangement 12 is decreased by switchingthe on/off valves 118, 120, wherein, by switching the first on/off valve118 from its flow-through position to its blocked position, the dynamicpressure produced by the hydraulic pump 54 and the flow control valve 16is blocked compared to the pressure chamber 68 of the firstpiston/cylinder arrangement 12, while switching the second on/off valve120 from its blocked position to its flow-through position connects thepressure chamber 68 of the first piston/cylinder arrangement 12 to thereservoir 58. As a result, the braking pressure in the pressure chamber68 of the first piston/cylinder arrangement 12 to the reservoir 58 isdecreased until it drops below the deceleration threshold so that thewheel that was originally locked begins to rotate again. When the wheelthat was originally locked begins to rotate again, the electro-magneticcontrol of the valves 114, 118, and 120 is interrupted so that thevalves 114, 118, and 120 each return to their pre-stressed normalposition and a braking force that is proportional to the actuating forceis applied in the brake caliper 10 by power brake circuit A and brakecircuit B as described referring to FIG. 1. A regulating processinitiates that continues through the anti-lock braking, whereby nonotable regulating pressure surges have a retroactive effect on thebrake pedal 2 because of the described structure of the power-assistedbraking system with the valves 114, 118, and 120. In particular,produced independent from the switch position of on/off valves 118 and120 in power brake circuit A is the dynamic pressure, which thereby isreported back unchanged through the control circuit C to the brake pedal2 so that there is good pedal responsiveness, even during ABS operationof the power-assisted braking system.

In accordance with the third embodiment example illustrated in FIG. 4,the power-assisted braking system per FIG. 1 or FIG. 3 is provided withadditional components in order to make possible a traction controlsystem (ASR) and a vehicle dynamics control system. The partscorresponding to the parts in FIGS. 1 and 3 are labelled with the samereference numbers and are not re-explained in the following. FIG. 4illustrates the power-assisted braking system with ASR and vehicledynamics control system in its unactuated mode.

Principally, the effect of a traction control system is that when amotor vehicle is being driven, if, for example, the coefficients offriction of a driven wheel to the ground are too low (that is, if anacceleration threshold is exceeded), said wheel is braked until it fallsbelow a second acceleration threshold, so that the wheel is again movingin a reliable traction range. In contrast, in a vehicle dynamics controlsystem, which is principally tailored to maintain the longitudinalstability of the vehicle (which can be measured by the yaw angle betweenthe longitudinal axis of the vehicle and the momentary direction oftravel), e.g. braking of one or more wheels is used to correct yawingmoments that could lead to the vehicle swerving or skidding in criticaldriving conditions like, e.g., understeering, oversteering, or brakingduring rapid travel of a curve.

This means that for such a control system it must be possible tomodulate the braking pressure on individual wheels. This is providedusing the previously described valves 118 and 120, which are providedfor every wheel, as well as an additional electro-magnetic control ofthe flow control valve 16. To facilitate electro-magnetic control, theflow control valve 16 has an electro-magnet 126 that can be controlledindependently from the actuation of the brake pedal 2. Theelectro-magnet 126 is connected to the control piston 40 of the flowcontrol valve so that the control piston 40 can be moved axially bycontrolling the electro-magnet 126, for which purpose the electro-magnet126 is designed as a proportional magnet having the correspondingcurrent/force characteristic. This means the control piston 40 can bemoved by the electromagnet 126 in a controllable manner depending onforce, whereby the valve passage area 48 of the flow control valve 16decreases when a current is applied to the electro-magnet 126, i.e.,when there is greater force. It furthermore means that it enlarges whenthe current is removed as a result of the force of the pullback spring50 of the flow control valve 16 as well as the dynamic pressure actingon the valve regulator body 36.

As a result, with the appropriate sensor technology, it is possible toautomatically counteract the over-reactions or erroneous reactions thathave already been introduced by the driver during operation of the motorvehicle before the driver becomes aware of the critical situation andactuates the brake pedal 2. It is understood that in the brakingsituation addressed here, in which there is a traction control andvehicle dynamics control system and in which the driver is not an activeparticipant, no pressure builds up in brake circuit B; this is why therelief valve 114 does not have to be controlled and remains in itsnormal position.

In accordance with the fourth embodiment example illustrated in FIG. 5,the braking system is designed as a power braking system. The partscorresponding to the parts in FIG. 1 are labelled with the samereference numbers and are not re-explained in the following. FIG. 5illustrates the power braking system in its unactuated mode.

The power braking system in accordance with FIG. 5 also has, connectedto the brake pedal 2, the master cylinder 4, which has two pressurechambers 6, 8, of which the first pressure chamber 6 hydraulicallycontrols the flow control valve 16 through which flows pressure mediumduring servo-force braking in order to adjust its regulatingcross-section, i.e., its valve passage area 48, to produce a defineddynamic pressure. The control circuit C therefore corresponds to theembodiment examples described in the preceding with respect to structureand function and therefore does not require repeated, more detaileddescription.

The dynamic pressure produced in the flow control valve 16 applies abraking force to a piston/cylinder arrangement 12', hydraulicallyconnected to the flow control valve 16 during servo-force braking, of abrake application element in the form of a brake caliper 10', which, incontrast to the embodiments described in the preceding, has only asingle piston/cylinder arrangement 12'. The brake caliper 10' (afloating caliper in the illustration), like the embodiments described inthe preceding, is provided brake linings 78 that are pressed against thebrake disk 80 in a known manner when pressure is applied in the pressurechamber 68' of the piston/cylinder arrangement 12' by its piston 70'.

Furthermore, provided between the pressure line 66 of power brakecircuit A and the pressure line 72 of brake circuit B is an additionalseparator piston/cylinder arrangement 150, the separator piston 152 ofwhich hydraulically separates a servo-pressure chamber 154, positioned(in FIG. 5) to the right of the separator piston 152, from an actuatingpressure chamber 156, positioned (in FIG. 5) to the left of theseparator piston 152. The servo-pressure chamber 154 is hydraulicallyconnected to the piston/cylinder arrangement 12' of the brake caliper10' by a pressure line 158, while the actuating pressure chamber 156 isattached by the pressure line 72 to the second pressure chamber 8 of themaster cylinder 4, so that even if power brake circuit A fails, abraking force for power-assisted braking can be applied to the brakecaliper 10' through the pressure in the pressure line 72 of brakecircuit B and thereby in the actuating pressure chamber 156 of theseparator piston/cylinder arrangement 150 through the separator piston152; this is described in more detail in the following.

In detail, the separator piston/cylinder arrangement 150 has apreferably cylindrical housing 160 having a cylinder bore 162 thatcontains the separator piston 152, which can be actuated on two sidesand which can move along its longitude. A port 164 for the pressure line158 leading to the piston/cylinder arrangement 12' is provided at the(in FIG. 5) right-hand face of the housing 160, and at the (in FIG. 5)left-hand face of the housing 160 a port 166 for the pressure line 72leading to the master cylinder 4 is formed, while a port 168 for thepressure line 66 leading to the flow control valve 16 is providedapproximately in the center of the perimeter of the housing 160.

In the axial direction in an approximately a central cross-section, theseparator piston 152 has a smaller diameter that, along with thecylinder bore 162 and the cross-sections of the separator piston 152that have a greater diameter and that are located to either side of thecenter cross-section, defines an additional pressure chamber 170. Theadditional pressure chamber 170 is hydraulically separated from theservo-pressure chamber 154 and the actuating pressure chamber 156 (inFIG. 5) to the right and left by means of sealing elements 172 (likeO-rings) provided in circumferential grooves in the cross-sections ofthe separator piston 152 that have a larger diameter.

Furthermore, the separator piston 152 has a central valve 174, known inand of itself, in the braking hydraulics and is pre-stressed into theactuating pressure chamber 156 by means of a restoring spring 176contained in the servo-pressure chamber 154. The centric central valve174 has connected to a valve tappet 178 a valve sealing body 180 that ispre-stressed in the direction of the actuating pressure chamber 156 bymeans of a valve spring 182 against a sealing seat 184 joined to theseparator piston 152. Finally, the separator piston 152 is provided arecess 186 that is gripped by a catch 188 attached to the interiorcircumferential wall of the housing 160.

In the normal position of the separator piston 152 illustrated in FIG.5, said piston is pre-stressed by the force of the restoring spring 176against the catch 188. At the same time, the valve tappet 178 isadjacent to the catch 188, whereby the valve sealing body 180 is held ata distance from the sealing seat 184 on the separator piston 152 by thevalve tappet 178 against the force of the valve spring 182 so that thecentral valve 174 is open.

As a result, the additional pressure chamber 170 of the separatorpiston/cylinder arrangement 150 is hydraulically connected to itsservo-pressure chamber 154 by the open central valve 174, so that duringoperation the servo-pressure in power brake circuit A occurring in thepressure line 66 acts on the piston/cylinder arrangement 12' of thebrake caliper 10' through the additional pressure chamber 170 of theseparator piston/cylinder arrangement 150, the open central valve 174,the servo-pressure chamber 154, and the pressure line 158, while inbrake circuit B the second piston 22 of the master cylinder 4 ishydraulically connected to the separator piston 152 through the secondpressure chamber 8 of the master cylinder 4, the pressure line 72, andthe actuating pressure chamber 156 of the separator piston/cylinderarrangement 150.

At this point it should be remarked that, for the sake of simplicity,FIG. 5 only illustrates a basic embodiment of the power braking systemhaving a brake caliper 10' for one wheel. Additional brake calipers areconnected analogously through the branches 66', 72' from the pressurelines 66, 72, each brake caliper being allocated a separatorpiston/cylinder arrangement. Likewise, instead of the brake caliper,commensurate with the requirements of the situation, the wheel brakecylinders can also be hydraulically controlled by conventional drumbrakes by means of the braking system.

The manner in which the fourth embodiment example functions is describedin the following to the extent that it differs from the first embodimentexample that was described using FIG. 1 to illustrate. During normalbraking (i.e., during servo-force braking), the valve passage area 48 ofthe flow control valve 16 is adjusted by control circuit C as describedand illustrated by FIG. 1 depending on the pressure produced in thefirst pressure chamber 6 of the master cylinder 4 when the brake pedal 2is pressed downward. When the hydraulic pump 54 is running, the dynamicpressure produced by forced flow through the valve passage area 48 inthe flow control valve 16 is applied through the pressure line 66 ofpower brake circuit A, the port 168 of the separator piston/cylinderarrangement 150, the additional pressure chamber 170, the open centralvalve 174, the servo-pressure chamber 154, the port 164, and thepressure line 158 in the piston/cylinder arrangement 12' of the brakecaliper 10'.

At essentially the same time or at a slight delay the pressure producedin the second pressure chamber 8 of the master cylinder 4 is appliedthrough the pressure line 72 of brake circuit B and the port 166 of theseparator piston/cylinder arrangement 150 in its actuating pressurechamber 156.

The separator piston 152 remains in its normal position at the catch188, illustrated in FIG. 5, because of the essentially simultaneouspressure actuation of the pressure lines 66, 72, i.e., the subsequentbuild up in pressure in brake circuit B compared to power brake circuitA. However, this requires that the effective hydraulic surfaces of theseparator piston 152 and/or the operating pressure level in controlcircuit C or brake circuit B compared to power brake circuit A areselected so that the forces affecting the separator piston 152 canceleach other out or the resulting force has an effect in the direction ofthe actuating pressure chamber 156, smaller differences beingcompensated by the pre-stressed force of the restoring spring 176. As aresult, the braking force is applied to the brake disk 80 solely by thedynamic pressure produced in the flow control valve 16 through thepiston/cylinder arrangement 12' of the brake caliper 10', an appropriateresponse from the dynamic pressure to the brake pedal 2 occurringthrough the pressure columns of the control circuit C between the flowcontrol valve 16 and the master cylinder 4, this permitting responsiveactuation of the brake pedal 2.

If there is no pressure in power brake circuit A in the pressure line 66as a result of the hydraulic pump 54 failing or as a result of a leak inthe separator piston/cylinder arrangement 150 in power brake circuit Aor control circuit C, the pressure in the actuating pressure chamber 156as a result of the brake pedal 2 being depressed pushes the separatorpiston 152 against the force of the restoring spring 176 out of itsnormal position away from the catch 188 into the servo-pressure chamber154. In so doing, the valve tappet 178 is released from the catch 188 sothat the valve sealing body 180 is pressed against the sealing seat 184by the valve spring 182. Accordingly, the central valve 174, now closed,hydraulically separates the additional pressure chamber 170 from theservo-pressure chamber 154. At the same time, as a result of theseparator piston 152 being displaced, the required power-assistedbraking pressure builds up in the servo-pressure chamber 154 and isapplied through the port 164 and the pressure line 158 to thepiston/cylinder arrangement 12' of the brake caliper 10' to produce abraking force on the brake disk 80 for power-assisted braking. When theseparator piston 152 is pushed further (in FIG. 5) to the right, theseparator piston 152 strikes the catch 188 at the end of the recess 186,which limits the power-assisted braking pressure to a maximum value andprevents the central valve 174 from driving onto the bottom of thehousing 160.

The reaction force produced during power braking at the piston 70' ofthe piston/cylinder arrangement 12' of the brake caliper 10' is reportedback through the pressure columns between the piston 70' and theseparator piston 152 and through the pressure columns between theseparator piston 152 and the second piston 22 of the master cylinder 4to the brake pedal 2 so that even during assisted-power braking there isresponsive actuation of the brake pedal 2 that reflects the actualbraking pressure the brake caliper 10'.

In accordance with FIG. 5, the reservoir 58 for the pressure medium,separate from the balance tank 26 of the master cylinder 4, ispreferably provided with a fill level sensor 190 that is switched withthe control of the drive of the hydraulic pump 54, like an electro-motoror an electro-magnetic coupling (not shown). Should there be a failureof mechanical origin (for instance, as a result of the pressure line 66or the pressure line 158 rupturing), the decrease in the pressure mediumin the reservoir 58 caused while the hydraulic pump 54 is operating isdetected by the fill lever sensor 190. Depending on a fill level signalproduced by the fill level sensor 190, the drive of the hydraulic pump54 is then turned off in order to prevent the reservoir 58 from beingpumped until it is empty. Even given a rupture in the pressure line 158of the affected brake caliper 10', and especially if the servo-pressurefails as a result of the drive for the hydraulic pump 54 being turnedoff, power-assisted braking is possible in brake circuit B through theother functional brake application elements (not shown), e.g., the brakecalipers or wheel brake cylinders that are connected by the branch 72'.It is useful for brake circuit B and control circuit C to be suppliedpressure medium from the reservoir 58 independently through the balancetank 26 provided at the master cylinder 4. FIG. 6A illustrates a firstvariant 150' of the separator piston/cylinder arrangement 150 describedusing FIG. 5 as a reference; it can be used instead of the separatorpiston/cylinder arrangement 150 in accordance with FIG. 5.

The separator piston/cylinder arrangement 150' in accordance with FIG.6A differs from the separator piston/cylinder arrangement 150illustrated in FIG. 5 in that no central valve is provided in theseparator piston 152', so that the separator piston/cylinder arrangement150' can be designed to be more cost-effective. The port 168' in thecircumferential wall of the housing 160' is attached in such a way thatthe pressure line 66 is connected hydraulically directly to theservo-pressure chamber 154 when the separator piston 152' ispre-stressed in its normal position by means of the restoring spring176.

If, as described above, there is an unintentional drop in servo-pressurein the pressure line 66, the pressure occurring in the actuatingpressure chamber 156 when the brake pedal 2 is pressed downward pushesthe separator piston 152' in FIG. 6A to the right; this causes thesealing element 172 facing the actuating pressure chamber 156 to travelover the port 168' so that the pressure line 66 is connected to theadditional pressure chamber 170 at the separator piston 152'. When theseparator piston 152' is pushed even further (in FIG. 6A) to the right,a braking pressure for power-assisted braking is built up in thepiston/cylinder arrangement 12' of the caliper 10', as described above.

FIG. 6B illustrates an economical second variant 150" of the separatorpiston/cylinder arrangement 150 described using FIG. 5 as a reference;it can be used instead of the separator piston/cylinder arrangement 150or 150' in accordance with FIG. 5. The separator piston/cylinderarrangement 150" in accordance with FIG. 6B does not have a centralvalve in the separator piston 152" either, and also differs from theseparator piston/cylinder arrangement 150' illustrated in FIG. 6A inthat the circumferential wall of the housing 160" is not provided with aport for the pressure line 66 and the separator piston 152" can beformed without additional pressure chambers. In this variant, thepressure line 66 is attached directly to the pressure line 158. Sincethe separator piston/cylinder arrangement 150" therefore cannot separatethe pressure line 66 from the pressure line 158, when pressure medium islost in power brake circuit A there can be no power-assisted brakingthrough the brake caliper 10' allocated to the separator piston/cylinderarrangement 150". On the contrary, power-assisted braking must occurover brake circuit B through the additional functional brake applicationelements that are connected by branch 72'. If control circuit C fails,or if hydraulic pump 54 in power brake circuit A fails and there is noloss of pressure medium, however, power-assisted braking through theseparator piston/cylinder arrangement 150" is possible.

In accordance with the fifth embodiment example illustrated in FIG. 7,the power braking system in accordance with FIG. 5 is providedadditional components in order to enable an anti-lock brake system(ABS). The parts corresponding to the parts in FIGS. 1, 3, and 5 arelabelled with the same reference numbers and are not re-explained in thefollowing. FIG. 7 illustrates the power braking system with ABS in itsunactuated mode.

Like the second embodiment example, in accordance with FIG. 7 the fifthembodiment example is provided a relief valve 114 in brake circuit Bwhich is inserted into the pressure line 72 between the master cylinder4 and the separator piston/cylinder arrangement 150" and which isconnected by a relief line 116 to the reservoir 58. Here again there isa total of only one relief valve 114 in the braking system. The reliefvalve 114 connects the actuating pressure chamber 156 of the separatorpiston/cylinder arrangement 15" to the second pressure chamber 8 of themaster cylinder 4 or alternatively to the reservoir 58, it beingpre-stressed in the position that connects the actuating pressurechamber 156 of the separator piston/cylinder arrangement 150" to thesecond pressure chamber 8 of the master cylinder 4.

Power brake circuit A is furthermore provided with two on/off valves118, 120 for each wheel to be controlled, as in the second embodimentexample. The first on/off valve 118 is inserted into the pressure line66 to the piston/cylinder arrangement 12' of the brake caliper 10' andis pre-stressed in such a way that in its normal position it connectsthe pressure line 52 between the hydraulic pump 54 and the flow controlvalve 16 to the pressure line 158 between the separator piston/cylinderarrangement 150" and the piston/cylinder arrangement 12' of the brakecaliper 10'. Branching off between the first on/off valve 118 and thepressure line 158 is a line 122 that opens into a collective return line124 connected to the reservoir 58. The second on/off valve 120 isinserted into the line 122 and is pre-stressed in its closed position.

During ABS operations, if a braking force is introduced into the brakingsystem through the brake pedal 2, sensing technology (not shown) detectsthat braking pressure applied through power brake circuit A or brakecircuit B in the brake caliper 10' is causing the braked wheel (notshown) to lock. The braking pressure is now appropriately adjusted byelectromagnetic control of the valves 114, 118, and 120. In addition,the relief valve 114 is switched to its position connecting theactuating pressure chamber 156 of the separator piston/cylinderarrangement 150" to the reservoir 58, so that the pressure occurringbetween the second pressure chamber 8 of the master cylinder 4 and therelief valve 114 is locked in, while the pressure in the actuatingpressure chamber 156 is decreased to zero by the relief line 116 to thereservoir 58 so that the separator piston 152" is held in its normalposition.

In power brake circuit A, the braking pressure in the pressure chamber68' of the piston/cylinder arrangement 12' of the brake caliper 10' isreduced by switching the on/off valves 118, 120. In doing so, thedynamic pressure produced by means of the hydraulic pump 54 and the flowcontrol valve 16 is blocked by switching the first on/off valve 118 fromits flow-through position to its blocked position with respect to thepressure chamber 68' of the piston/cylinder arrangement 12', while thepressure chamber 68' is connected to the reservoir 58 by switching thesecond on/off valve 120 from its blocked position to its flow-throughposition. As a result, the braking pressure in the pressure chamber 68'decreases until it falls below the deceleration threshold, at whichpoint the wheel that was originally locked begins to rotate again.

When the wheel that was originally locked begins to rotate again, thevalves 114, 118, and 120 are no longer controlled electro-magnetically,so that they each return to their pre-stressed normal position and abraking force that is proportional to the actuating force is applied inthe brake caliper 10' by power brake circuit A as described referring toFIG. 5.

A regulating process initiates that continues through the anti-lockbraking. The described structure of the braking system with the valves114, 118, and 120 prevents detectable regulating pressure surges fromretroactively affecting the brake pedal 2. In particular, independent ofthe switch position of the on/off valves 118 and 120, the dynamicpressure is produced in power brake circuit A that is reported backunchanged through the control circuit C to the brake pedal 2 so thatthere is good pedal responsiveness, even during ABS operation of thebraking system.

In the selected circuit arrangement, in accordance with which each brakeapplication element is allocated a separator piston/cylinder arrangement150", each individual brake application element is protected, regardlessof the distribution of braking pressure, so that if said brakeapplication element fails, the remaining brake application elementsremain fully functional. If the hydraulic pump 54 fails, power-assistedbraking is effected through brake circuit B, as described previouslyreferring to FIG. 5.

Should there be a leak between the on/off valve 118 and the actuatingpressure chamber 156 of the separator piston/cylinder arrangement 150"or piston/cylinder arrangement 12' of the brake caliper 10', after thefill level sensor 190 detects a decrease in the pressure medium in thereservoir 58, the hydraulic pump 54 does not have to be adjusted ifmovement by the separator piston 152" of the separator piston/cylinderarrangements 150" can be detected by means of an activating contact orsensor (not shown). Namely, if there is such a leak, the pressure in theservo-pressure chamber 154 of the separator piston/cylinder arrangement150" of the affected brake circuit is decreased, which results in theaffected separator piston 152" being displaced because of the pressurein the actuating pressure chamber 156. After sensing that the affectedseparating piston 152" has moved, the allocated on/off valve 118 merelyneeds to be switched to its blocked position so that the other brakecircuit connected by branch 66' can continue to be supplied pressureproduced by the hydraulic pump 54 and the flow control valve 16.

At this point it should be remarked that the braking system shown inFIG. 7 does have variant 150" of the separator piston/cylinderarrangement illustrated in FIG. 6B; however, the separatorpiston/cylinder arrangement 150 or 150' illustrated in FIGS. 5 and 6A,respectively, could also be used.

In the first variant of the fifth embodiment example, illustrated inFIG. 8, only those features that differ from the fifth embodimentexample will be described in the following. Several brake applicationelements 10' (for example, of an axle circuit or a diagonal circuit) areconnected by the pressure line 158 or its branch 158' to a separatorpiston/cylinder arrangement 150 in order to jointly protect thecorresponding brake application elements 10' from servo-pressurefailure. Here, again, the variants 150' or 150" can be used instead ofthe separator piston/cylinder arrangement 150.

Furthermore, for every brake application element 10', two on/off valves118, 120 are provided; of these, the on/off valve 118 is inserted intothe pressure line 158 between the servo-pressure chamber 154 of theseparator piston/cylinder arrangement 150 and the pressure chamber 68'of the piston/cylinder arrangement 12' of the brake application element10'--in other words, behind the separator piston/cylinder arrangement150 (as seen from the flow control valve 16 or the hydraulic pump 54).This means that the relief valve provided in accordance with FIG. 7 canbe omitted because the separator piston 152 of the separatorpiston/cylinder arrangement 150 is actuated by the pressure produced inthe flow control valve 16, even during anti-locking control, and istherefore held in its normal position.

FIG. 9 shows a second variant of the fifth embodiment example that willonly be described in the following in terms of its features that differfrom the first variant of the fifth embodiment example in accordancewith FIG. 8. In accordance with FIG. 9, in addition to the separatorpiston/cylinder arrangement 150.1, a second separator piston/cylinderarrangement 150.2 is inserted parallel to it so that the brakeapplication elements 10' of each circuit, e.g., each axle or diagonalcircuit, are each jointly protected from servo-pressure failure. Thesecond separator piston/cylinder arrangement 150.2 is attached with itsactuating pressure chamber 156 to the branch 72' of the pressure line72, while the servo-pressure chamber 154 of the second separatorpiston/cylinder arrangement 150.2 is hydraulically connected by thebranch 66' to the pressure line 66.

One skilled in the art will recognize that the circuit variants of thebraking system in accordance with FIGS. 1, 3, 5, and 7 through 9 can becombined as appropriate, depending on the number of wheels to be braked,the type of brake application elements provided for the wheels, safetyissues, and controls desired, whereby electro-magnetic control of theflow control valve 16 (electromagnet 126) for a traction control systemor vehicle dynamics control system, as described referring to FIG. 4,can also be provided in the circuit variants of FIGS. 5 and 7 through 9.

A braking system is disclosed that has a manually-actuatable mastercylinder having two pressure chambers. The first pressure chamberhydraulically controls a flow control valve through which flows thepressure medium in servo-force braking in order to adjust its regulatingcross-section to produce a defined dynamic pressure that is applied to afirst piston/cylinder arrangement of a brake application element that isconnected to a flow control valve to produce a braking force. The secondpressure chamber can be connected to a second position/cylinderarrangement of the brake application element so that a braking force canbe applied if the servo-force fails. Also, instead of the secondpiston/cylinder arrangement, a separate separator piston/cylinderarrangement can be provided that is inserted between the second pressurechamber and the piston/cylinder arrangement. Then sufficient brakingforce is produced both in servo-force braking and if the servo-forcefails, and an appropriate response by the braking forces to the mastercylinder is effected so that there is always good actuatingresponsiveness.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the scope and spiritof the invention as set forth in the appended claims.

I claim:
 1. A hydraulic braking system, especially for motor vehicles,comprising:an actuating device; a master cylinder connected to saidactuating device, said master cylinder including a first pressurechamber and a second pressure chamber; a flow control valve having aregulating cross-section, through which flows a pressure medium duringservo-force braking, hydraulically connected to said first pressurechamber so that said first pressure chamber hydraulically controls saidflow control valve by adjusting the regulating cross-section to producea defined dynamic pressure to produce a braking force; a brakeapplication element including a piston/cylinder arrangement connected tosaid flow control valve; a separator piston/cylinder arrangement havinga servo-pressure chamber connected hydraulically to said flow controlvalve; and an actuating pressure chamber separated from saidservo-pressure chamber by a separator piston, said actuating pressurechamber being hydraulically connected to said second pressure chamber ofsaid master cylinder to apply a braking force to said brake applicationelement through said separator piston if said servo-force braking fails.2. The braking system in accordance with claim 1, wherein said separatorpiston/cylinder arrangement has an additional pressure chamberconcentrically surrounding said separator piston, said additionalpressure chamber being hydraulically separate from said actuatingpressure chamber.
 3. The braking system in accordance with claim 2,wherein said additional pressure chamber has a port for the dynamicpressure and said separator piston has a central valve pre-stressed in aclosed position that is opened in said normal position of said separatorpiston in order to connect said additional pressure chamber to saidservo-pressure chamber and to apply the dynamic pressure to saidpiston/cylinder arrangement of said brake application element.
 4. Thebraking system in accordance with claim 2, wherein said servo-pressurechamber is hydraulically separate from said additional pressure chamberand has a port for the dynamic pressure through which the dynamicpressure is applied in said normal position of the separator piston tosaid piston/cylinder arrangement of the brake application element andwhich the separator piston can travel over if the servo-force fails suchthat the port is closed with respect to the servo-pressure chamber andconnected to the additional pressure chamber.
 5. The braking system inaccordance with claim 1, wherein said separator piston/cylinderarrangement further comprises a sensor to detect movement of saidseparator piston.
 6. The braking system in accordance with claim 1,further comprising:a relief valve positioned between said secondpressure chamber of said master cylinder and said actuating chamber ofsaid separator piston/cylinder arrangement, said relief valvealternatively connecting said actuating pressure chamber to said secondpressure chamber or to a reservoir; and a first on/off valve and asecond on/off valve arranged in a section that can be actuated by saiddynamic pressure said section between said flow control valve and saidservo-pressure chamber of said separator piston/cylinder arrangement,said first on/off valve alternatively interrupting a connection betweensaid flow control valve and said servo-pressure chamber, and said secondon/off valve alternatively connecting said servo-pressure chamber tosaid reservoir.
 7. The braking system in accordance with claim 1,further comprising:a first on/off valve and a second on/off valvelocated between said servo-pressure chamber of said separatorpiston/cylinder arrangement and said piston/cylinder arrangement of saidbrake application element in a section that can be actuated by saiddynamic pressure, said first on/off valve alternatively interrupting theconnection between said servo-pressure chamber and said piston/cylinderarrangement, and said second on/off valve is constructed and arranged toalternatively connect said piston/cylinder arrangement to a reservoir.8. The braking system in accordance with claim 1, wherein said flowcontrol valve comprises:a ball check valve including a spherical valveregulator body and an annular sealing seat; and a valve passage arealocated between said spherical valve regulator body and said annularsealing seat, said pressure medium flowing through said valve passagearea when said dynamic pressure is produced, said ball check valveregulating said cross-section of said flow control valve by adjustablyenlarging or decreasing said valve passage area.
 9. The braking systemin accordance with claim 1, wherein said flow control valve furthercomprises:an electromagnet constructed and arranged to adjust saidcross-section of said flow control valve independent of said actuatingdevice.
 10. The braking system in accordance with claim 1, wherein saidseparator piston is pre-stressed in a normal position in the directionof said actuating pressure chamber by using a restoring spring so thatsaid separator piston remains in said normal position during servo-forcebraking.